Do a google images search for 1991 Diamond Back Axis. Not saying that's what it is (I had one and it was a different colour), but all the distinctive features such as the stay bridges are the same.
Just walked past my 91 Axis - it had no rack mounts on the seat stays like that one, but my 91 Apex does. Also, that year Axis (and Axis Team) had bigger diameter tubes (takes a 30.4 seatpost, for example) - downtube and top tube look a far bit bigger than the normal steel frames from that year (like my AVR framed 91 Apex, which takes a 27.0 seatpost).
I'd agree, that DB used those outer sleeves / butting at the top seat tube junction, and I suppose it could be another DB frame from around that era - it's just not a 91 Axis.
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